The present invention generally relates to superconducting circuits and more particularly to a superconducting quantum interference device having an extended dynamic range for measurement of magnetic field.
Conventionally, various magnetometers have been employed for the measurement of weak magnetic fields. Such magnetometers include Hall-effect devices, flux gate devices, and superconducting quantum interference devices. Among others, the superconducting quantum interference magnetometers using the superconducting quantum interference device (abbreviated hereinafter as SQUID) for the magnetic field detection are capable of measuring extremely feeble magnetic fields (less than 10.sup.-10 T) and find applications in various fields such as biomagnetic measurements, detection of gravitational radiations, and various geophysical applications.
Conventionally, the SQUID magnetometer comprises a SQUID sensor for detecting the magnetic field and a feedback loop for measuring the magnitude of the detected magnetic field. The SQUID sensor may be a d.c. biased device as disclosed by Ketchen (Ketchen, M. B. "Integrated Thin Film, DC SQUID Sensors," IEEE Trans. Magn., vol.MAF-23, no. 2, pp. 1068-1071, 1987.) or a d.c. biased device with Josephson comparator which produces pulse that depends on an analog output of the d.c. biased device as disclosed by Drung (Drung, G., "Digital Feedback Loops for d.c. SQUIDs," Cryogenics, vol. 26, pp. 623-627, 1986), and used in combination with an external feedback circuit that measures the magnitude of the detected magnetic field. As the SQUID magnetometer employs the SQUIDs that operate only at a temperature below the critical temperature of normal conduction-superconduction for the magnetic sensor, the essential part of the magnetometer including the SQUID sensor is always immersed in a cooling medium typically of liquid helium of which boiling temperature is 4.2.degree. K. On the other hand, the bulky external feedback circuit has been provided outside the low temperature system and operated at the room temperature. Because of this, the conventional SQUID magnetometers have suffered from the problem of excessive consumption or evaporation of liquid helium caused by the penetration of heat into the low temperature system through the wiring fixture connecting the SQUIDs and the feedback circuit.
On the other hand, the applicants of the present invention have previously proposed a SQUID magnetometer system wherein the SQUID sensor and the feedback circuit are integrated on a single chip (Fujimaki, N. et al. "A Single-Chip SQUID Magnetometer," IEEE Trans. Electron Devices vol. 35, no. 12, December 1988). This prior art system has an advantageous feature in that the problem of excessive consumption or evaporation of helium due to the penetration of heat is successfully minimized by immersing the entire system in the liquid helium container. Further, this magnetometer produces the output in the form of digital output pulse which is advantageous in view point of increased signal-to-noise ratio and easiness in processing the output data by digital processing systems.
FIG. 1 shows the construction of this SQUID magnetometer proposed previously by the applicants. The magnetometer comprises a pickup coil 1 for detecting a magnetic flux .PHI..sub.x and a superconducting path 4 connected to the pickup coil 1. In the superconducting path 4, there are provided superconducting windings 2 and 3 of which function will be described later, and the pickup coil 1 and the superconducting windings 2 and 3 form a closed superconducting circuit with the superconducting path 4.
In the vicinity of the superconducting winding 2, there is provided another superconducting winding 11 that forms a closed interferometer loop 10 together with Josephson junctions J1 and J2. Thereby, respective first ends of the Josephson junctions J1 and J2 are connected commonly with each other to the superconducting ground plane. On the other hand, respective second ends of the Josephson junctions J1 and J2 are connected to corresponding ends of the winding 11. The winding 11 is so provided to establish a magnetic coupling M2 with the winding 2 in the superconducting path 4. Further, the end of the Josephson junction J1 that is connected to the superconducting winding 11 is connected to a power source 12 that supplies an a.c. drive current to the Josephson junctions J1 and J2. Thereby, the Josephson junctions J1, J2, the superconducting winding 2, and the superconducting winding 11 form a SQUID sensor 9 of which output is obtained at the node connecting the Josephson junction J1 and the winding 11.
The operation of the SQUID sensor 9 is as follows. Upon incidence of a magnetic flux .PHI..sub.x to the pickup coil 1, an induction current flows through the superconducting path 4 which in turn creates a magnetic field at the superconducting winding 2. This magnetic field interacts with the superconducting winding 11 and causes to flow a current through the Josephson junctions J1 and J2. On the other hand, the SQUID sensor 9 has a threshold characteristic as shown in FIG. 2 wherein either or both of the Josephson junctions J1 and J2 experience a transition from the zero-voltage state characterized by the zero voltage against a finite current, to the finite voltage state characterized by a finite voltage against the finite current when the supplied drive current has crossed a threshold level. This threshold level changes with the induction current that flows through the superconducting path 4 and hence with the incident magnetic flux .PHI..sub.x, and thereby the threshold level changes according to a threshold characteristic curve as shown in FIG. 2 by TH1, TH2, TH1' and TH2'.
In operation, the SQUID sensor 9 is driven with the a.c. drive current of which magnitude is set nearly the threshold level that is defined with respect to the case where there is no incident magnetic flux .PHI..sub.x. Thus when there is no incident magnetic flux, the Josephson junctions J1 and J2 in the interferometer loop 10 remain in the zero-voltage state and no output voltage will appear on the output terminal of the SQUID sensor 9. On the other hand, when there is an input flux .PHI..sub.x as shown in FIG. 3A, the operational point of the SQUID sensor is moved to a point A in FIG. 2 in correspondence to the negative part A of the waveform in FIG. 3A and crosses the threshold curve at a point C' in response to the negative peak of the drive current in the first half cycle of the a.c. drive current. Thereby, the Josephson junction J1 cases a transition to the finite voltage state that is immediately followed by the Josephson junction J2. In response to the transition of the Josephson junctions J1 and J2 thus caused, a negative output voltage appears on the output terminal because of the finite resistance of the interferometer loop 10. Subsequently, the drive current swings back. The Josephson junctions J1 and J2 return to the zero-voltage state again when the drive current decreases less than the point C' due to a large hysteresis of the current-voltage curve of the device. Thereby, the output voltage at the output terminal returns to zero and thus, the output of the SQUID sensor takes the form of a voltage pulse. In the following positive half cycle, the transition to the finite voltage state does not occur, as the threshold level TH1 for this case is shifted in the positive direction and the crossing of the threshold level TH1 by the operating point does not occur even in the peak of the drive current.
In the case when the incident flux .PHI..sub.x has a direction opposite to the foregoing case as shown in the part B of FIG. 3A, on the other hand, the operating point crosses at a point C shown in FIG. 2 near the positive peak of the a.c. drive current. No crossing of the negative threshold level TH1' occurs in this case. Thereby a positive voltage pulse appears on the output terminal.
Thus, the SQUID sensor of FIG. 1 produces a voltage pulse having a polarity that represents the direction or polarity of the incident magnetic flux as shown in FIG. 3B. On the other hand, the output voltage pulse of the SQUID sensor as shown in FIG. 3B does not provide any information about the magnitude of the magnetic flux thus detected. FIG. 3C shows the determination of direction of the detected magnetic flux. In this process, the positive pulses and negative pulses are counted for a predetermined period and the number of negative pulses is subtracted from the number of the positive pulses. Thereby, the difference in the number of positive and negative pulses in a unit time, n.sup.+ -n.sup.-, changes with the input as shown by the solid line. However, because of the fluctuation such as the thermal noise, the output pulse changes as shown by the dotted line, reflecting the stochastic process.
FIGS. 4A and 4B show the construction of a superconducting feedback circuit 8 which is integrated on the single chip SQUID magnetometer of FIG. 1 and used in combination with the SQUID sensor for determination of the magnitude of the magnetic flux. The feedback circuit 8 comprises a superconducting winding 23 having a first end connected to the SQUID sensor 9 via a resistor 24 and a second end connected to a first end of another superconducting winding 22 that establishes a magnetic coupling M3 with the winding 23. The superconducting winding 22 has another, second end at the side opposite from the first end, wherein both the first and second ends of the superconducting winding 22 are connected to the superconducting ground plane via respective Josephson junctions J3 and J4. Thereby, the winding 22 and the Josephson junctions J3 and J4 form a superconducting interferometer loop 21 similar to the superconducting interferometer loop 10 of the SQUID sensor 9, except that the winding 22 is directly connected to the winding 23. Further, the foregoing second end of the superconducting winding 23 is connected to a first end of a large superconducting winding 26 of which inductance value may be 10-30 .mu.H in the typical example. Further, the superconducting winding 26 has a second end that is connected, via a superconducting path, to another superconducting winding 7 that establishes a magnetic coupling M1 with the superconducting winding 3 in the superconducting path 4. Thereby, there is formed a magnetic flux storage loop 27 to be described later by the Josephson junctions J3, J4 and the superconducting windings 22, 23, 26 and 7.
The operation of the feedback circuit 8 is as follows. Referring to FIG. 4A, the output voltage pulse of the SQUID sensor 9 is supplied to the superconducting winding 23 in the form of an input current pulse Ic. This current pulse Ic is divided into a path flowing through the loop 21 and another path flowing through the winding 26, wherein the majority of the current flows through the loop 21 as a current Ig because of the large inductance value of the winding 26. In other words, the current Ig is nearly the same as the magnitude of the current pulse Ic.
FIG. 4B shows the operational characteristic of the feedback circuit 8. Because of the fundamental interferometer construction of the loop 21, the circuit 21 has a threshold characteristic similar to that described with reference to FIG. 2, wherein the threshold lines TH1, TH2, TH1' and TH2' are repeated regularly along the horizontal axis representing the current Ic. Such a repetition of the characteristic is designated in FIG. 4B as MODE0, MODE+1, MODE-1, . . . , in which the MODE0 represents the state where there is no flux quantum trapped in the loop 21, the MODE+1 represents the state wherein a single flux quantum having a first direction or polarity is trapped in the loop 21, and the MODE-1 represents the state wherein a single flux quantum having a second, opposite polarity is trapped in the loop 21.
Starting from the MODE0, the current Ig increases with the current pulse Ic in response to the incoming of the output pulse of the SQUID sensor 9, and the operational point moves along a straight line A-B in FIG. 4B. At the beginning, this current Ig mainly flows through the Josephson junction J3 because of the finite inductance value of the winding 22 connected in series to the Josephson junction J4. Thereby, the operational point crosses the threshold level TH1 of the MODE0 at a point J3 on the line A-B and the Josephson junction J3 causes a transition to the finite voltage state in response thereto. Simultaneously to the switching of the Josephson junction J3 thus caused, the path of the current Ig is now changed to flow through the Josephson junction J4. In response to this, the magnetic flux that accompanies the current Ig flowing through the Josephson junction J4 enters into the superconducting interferometer loop 21 at the Josephson junction J3 that is now in the finite voltage state in the form of magnetic flux quantum. In response to the entering of the flux quantum, the mode of the loop 21 changes to the MODE+ 1 and the Josephson junction J3 returns immediately to the zero-voltage state. It should be noted from FIG. 4B that the point J3 is located within the zero-voltage region of the MODE+1 mode.
Subsequent to the aforementioned process, the magnitude of the current Ic is decreased again in response to the falling edge of the output pulse, and thereby the operational point moves toward the origin O of the graph of FIG. 4B along the line A-B. During this process, the operational point crosses the threshold level TH2 of the MODE+1 at a point J4, and in response thereto, the Josephson junction J4 experiences a momentary transition to the finite voltage state similar to the Josephson junction J3. When this occurs, the flux quantum held in the superconducting loop 21 is transferred to the superconducting winding 26 of the storage loop 27. When a negative voltage pulse arrives from the sensor 9, an essentially same phenomenon occurs with the polarity reversed, along the line O-B. Thereby, a persistent current is induced in the winding 26 and the flux quantum is transferred to the superconducting storage loop 27. Thereby, the magnetic flux in the winding 26 increases or decreases stepwise in response to each entering of the flux quantum. Of course, in the case when the polarity of the flux quantum that is newly transferred to the loop 27 has a polarity that cancels the magnetic flux stored therein, the magnetic flux in the winding 26 is decreased stepwise.
After the transfer of the trapped flux quantum to the flux storage loop 27, the mode of the interferometer 21 returns to the MODE0. Thereby, the Josephson junction J4 returns to the zero-voltage state. It should be noted that the point J4 is located inside the MODE0 region and thus, the Josephson junction J4 should be in the zero-voltage state when the interference loop 21 has returned to the MODE0.
As described heretofore, the superconducting loop 21 forms a functional block 25, together with the superconducting winding 23, that writes a magnetic flux into the superconducting winding 26 of the storage loop 27 in response to the output pulse of the SQUID sensor 9. Because of this reason, the functional block 25, comprising the superconducting windings 22, 23 and 26 as well as the Josephson junctions J3 and J4, is called a "write gate."
FIGS. 5A and 5B show the foregoing sequence of storing the flux quantum in the storage loop 27, wherein the Josephson junction J3 causes the momentary transition from the zero-voltage state to the finite voltage state in response to the leading edge of the current pulse Ic while the Josephson junction J4 causes the momentary transition from the zero-voltage state to the finite voltage state in response to the trailing edge of the current pulse Ic. In response to the each transition of the Josephson junction J3 that is followed by a corresponding transition of the Josephson junction J4, a magnetic flux .PHI..sub.O corresponding to the foregoing flux quantum is added in the superconducting winding 26 of the storage loop 27, and the magnetic flux .PHI. stored by the superconducting winding 26 is increased or decreased stepwise as shown in FIG. 5B.
It should be noted that the flux storage loop 27 is coupled to the superconducting path 4 at the windings 3 and 7. Thereby, the persisting current in the loop 27 corresponding to the stored magnetic flux flows through the winding 7 as a feedback current IFB and the magnetic flux .PHI..sub.x is canceled out by the magnetic flux .PHI. step by step with increasing magnetic flux in the storage loop 27 until it disappears entirely. During this process, the output pulse of the SQUID sensor 9 is counted up by an external counting circuit and the number of counts thus obtained represents the magnitude of the incident magnetic flux .PHI..sub.x.
In this conventional SQUID magnetometer, there exists a problem in that the magnetic flux that can be stored in the storage loop 27 is limited even though the superconducting winding 26 is designed to have a large inductance. As a matter of fact, the dynamic range of the SQUID magnetometer of the foregoing prior art has been limited in the order of 10.sup.5 in terms of the number of flux quanta that corresponds to the magnetic flux the storage loop 27 can store.
FIG. 6 is a diagram for explaining the cause of this undesirable result.
It should be noted that each time a flux quantum is transferred to the storage loop 27, a minute persistent current flows through the loop 27. As will be understood from FIG. 4A, this persistent current contributes to the current Ig. Thus, the current Ig does not return to zero exactly once the magnetic flux corresponding to the flux quantum is entered to the storage loop 27, even when the current Ic is returned to zero thereafter. Although the magnitude of the persistent current per one flux quantum is small in view of the large inductance of the superconducting winding 26, the effect of this persistent current nevertheless becomes appreciable when a large number of flux quanta are transferred to the storage loop 27. It should be noted that it is exactly this current Ig that induces the counteracting magnetic field at the winding 7.
When this occurs, the origin O of FIG. 4B will shift in the upward or downward direction to a new origin O' or O" along the vertical axis as shown in FIG. 6, and the operational line O-A or O-B on which the operational point moves experience a corresponding shift in the downward direction or in the upward direction to a new line O'-A' or O"-B'. When the magnitude of the magnetic flux .PHI..sub.x under investigation is large, the line O'-A' may ultimately cross a point C defined in FIG. 6 as an intersection of the threshold line TH2 and the threshold line TH1' of the MODE+1. When this occurs, the operational point moving along the new line O'-A' in response to the current Ic will cross the threshold TH1' of the MODE+1 instead of the threshold TH2 thereof. Thus, when the operational point returns after crossing the threshold TH1 of the MODE0 and causing thereby the transition of the Josephson junction J3 that eventually causes the transition of the mode from the MODE0 to the MODE+1, the Josephson junction J3, not the Josephson junction J4, will cause the momentary transition to the finite voltage state upon the crossing of the operational point through the threshold TH1' of the MODE+1. In other words, the transition of the Josephson junction J4 will not occur in this process and the flux quantum that has entered into the loop 21 through the transition of the Josephson junction J3 may exit therefrom upon the transition of the Josephson junction J3 made after the first transition. Thereby, no storage of new flux quantum occurs anymore.
An exactly the same situation occurs also in the case where the line O-B has shifted in the upward direction to the line O"-B' that locates above a point D where the threshold line TH1 and the threshold line TH2' of the MODE-1 crosses. Thus, the conventional SQUID magnetometer of FIG. 1 suffers from a problem of limited dynamic range that the magnetometer is operational.